High quality free-standing (FS) GaN layers of thickness similar to 600 mu m were prepared by exploiting spontaneous separation mechanism. Study of the mechanism revealed that the separation took place inside the GaN layer and not from the sapphire substrate. The abrupt changes in growth conditions during the deposition process and thermal mismatch between GaN and sapphire are discussed as possible causes of such spontaneous separation of GaN layers. The substrates were prepared by overgrowing GaN templates using the HCl based hydride vapor phase epitaxy (HVPE). The obtained FS-GaN substrates had a dislocation density (DD) in the order of 10(6) cm(-2). Low temperature photoluminescence (PL) and reflection spectroscopy showed that the FS-GaN have good optical quality. The substrates were nearly stress free as revealed by X-ray diffraction (XRD), PL, and reflection spectroscopy. (C) 2010 Elsevier B.V. All rights reserved.